The microwave frequency range is in that portion of the electromagnetic spectrum where the wavelength is of the same order of magnitude as the characteristic size of the circuit carrying the electrical energies. The frequencies most often considered to be in that category lie approximately between 1 and 200 GHz. Through the use of microwave integrated circuits, the microwave frequency range has been widely employed in communications systems, radar systems, and in various other applications. Capacitors adapted to operate at microwave frequencies are an integral pan of microwave integrated circuits, finding use in filters, dividers and couplers, as well as in impedance matching, RF bypassing and DC blocking circuits. The metal-insulator-metal (MIM) capacitor structure is the most commonly employed capacitor type in microwave integrated circuits.
The MIM capacitor is a direct miniaturization of the conventional parallel plate capacitor. In monolithic microwave integrated circuits (MMICs), MIM capacitors are generally fabricated by sandwiching a thin layer of a low loss dielectric between two metal plates. The choice of lumped or distributed elements depends largely upon the frequency of operation. Lumped elements are suitable through X-band up to, perhaps, 20 GHz. Beyond this frequency range, distributed elements are generally preferred. It is difficult, however, to realize a truly lumped element, even at the lower frequencies, because of the parasitic to ground associated with thin substrates.
In FIG. 1A, there is illustrated a perspective view of a typical MIM capacitor 1 positioned on a semi-insulating substrate 2. The bottom plate 3 is comprised of a thin unplated metal while the dielectric layer 4 disposed thereon is comprised of a dielectric material having a sufficiently high dielectric constant to provide the desired capacitance. The dielectric material is typically Silicon Nitride (Si.sub.3 N.sub.4), although other materials can be employed. The top plate 6 generally comprises a thick plated conductor to reduce the loss in the capacitor. The top plate is generally connected to other circuitry via an air bridge which provides higher breakdown voltages.
When a MIM capacitor is used in a monolithic microwave integrated circuit (MMIC), two or more connections are generally made between the capacitor and other circuit elements. These multiport connections may be placed anywhere along the periphery of the capacitor as required by the design considerations of circuit layout compactness and flexibility. In the circuit shown in FIG. 1A, these connections are made by means of air bridges 7 and 8. A disadvantage of multiport connections to conventional capacitors, however, is that both collinear and non-collinear connections introduce performance degrading parasitic reactances in the current path of the circuit. FIGS. 1B and 1C exemplify two types of multiport capacitor connections. FIG. 1B depicts a straight feed configuration in which connections are made to the capacitor 1 at defined collinear ports 8a and 8b to minimize the level of discontinuity in the current path. FIG. 1C illustrates a bend-feed configuration in which circuit layout considerations preclude a collinear arrangement between defined ports 9a and 9b . Instead, the connections are made to the capacitor 1 at right angles, thereby resulting in very high discontinuity reactances and in a substantial degradation of performance.
Accordingly, it is an objective of the present invention to provide a MIM capacitor which is capable of accommodating non-collinear multiport connections while minimizing the level of added discontinuity reactances. It is also an objective of the present invention to provide a MIM capacitor which improves the performance of collinear multiport connections as well.